Cold and hollow electrode



5 P. LEMAlGRE-VOREAUX 2,913,293

COLD AND HOLLOW ELECTRODE Filed Dec. 9, 1957 [/v l/E/V 70/2 PIERRELmlmlazz-l oflz'fiux United States Patent COLD AND HOLLOW ELECTRODEPierre Lemaigre-Voreaux, Paris, France, assignor to Claude Paz etVisseaux, Paris, France Application December 9, 1957, Serial No. 701,671

Claims priority, application France December 19, 1956 1 Claim. (Cl.316-26) This invention relates to cold and hollow electrodes for gaseouselectric discharge lamps and is an improvement in or modification of theinvention described in the specification of my prior patent applicationNo. 562,565 filed January 31, 1956, titled Cold' and Hollow Electrodeand owned by the present assignee.

In the specification of the prior patent application above referred tothere is described a cold and hollow electrode for an electric dischargelamp containing gas and mercury vapour such electrode having aflixed tothe inner wall thereof at least one small piece comprising, mainly orsolely, one or more rare earth metals such as lanthanum in a metalliccondition. In such an electrode, the total area of the piece, or pieces,is small and remains small, as compared with the area of the inner wallof the electrode and preferably is smaller and remains smaller than onetenth of the area of such wall, at least during almost the whole life ofthe electrode.

In a copending patent application No. 670,714 filed July 9, 1957 isdisclosed an improved step in the formation process of electrodesaccording to the above patent application, such step comprising a heattreatment in an atmosphere of nitrogen.

An important object of the present patent application is to provideelectrodes of the kind disclosed in the patent application No. 562,565the voltage drop of which remains still longer relatively low.

The present invention is characterized in that the electrode, providedwith its piece, or pieces, comprising rareearth metal, is subjected toelectric discharges in an atmosphere of helium with an atmosphere of afew millimeters of mercury when cold. The helium used contains at most arelatively small proportion of impurities, especially as regards theoxygen content.

This improvement applies whatever may be the nature of the dischargeatmosphere wherein the electrode will normally be required to operatewhether such atmosphere contains helium or not, the latter case being byfar the most frequent occurrence.

Like nitrogen, used in the process described in the prior patentapplication No. 670,714, the employment of a helium atmosphere willreduce considerably the oxidation of the surface of the rare earth metalpieces as compared with a similar treatment efiected in traces of airwhich may remain after a moderate pumping. The use of helium as opposedto nitrogen offers the following advantage: it reduces considerably oreliminates entirely, depending upon its nitrogen content and upon thetreatments to which the electrode is subjected, the superficialnitrogenation of the lanthanum or other rare earth metal (includingcerium) or alloy. This nitrogenation would decrease the absorptioncapacity of the metal for gas and might result in an excessive evolutionof nitrogen if the electrode were used for too high currents in view ofits dimensions. Electrodes, treated according to the present inventionutilize better than those of the patent application No. 670,714 theadvantageous properties of the metals or alloys in question, while with-2,913,298 Patented Nov. 17, 1959 application No. 562,565, is describedhereinafter, by way of example, with reference to the accompanyingdrawing which shows said electrode in longitudinal section.

As described in the specification of the patent application No. 562,565,the electrode may comprise a cylinder 1 of nickel plated sheet .iron atthe ends of which two steatite pieces are located such pieces comprisinga bottom 7 and a ring 2 the latter incorporating an orifice 3 and acollar 5 which serves to protect the edge 4 of the cylinder 1. A currentlead-in wire 8 is welded to the cylinder 1 and one or more sections 6 oflanthanum wire have been welded inside the cylinder in the vicinity ofthe bottom piece 7.

After the various portions of the electrode have been assembled, theelectrode is subjected to a degassing and formation treatment, which, atthe same time, degasses the inner surface of the envelope inside whichsaid electrode is mounted. Such treatment may, for example, be asfollows:

Two similar electrodes are sealed in a vacuum-tight manner,respectively, at the ends of a glass tube provided with an exhaust tube.The tube is then connected, by means of exhaust tube, to a vacuum pump,so as to drain it fairly completely of air. The tube is then filled withcommercial helium under a pressure of the order of one or twomillimeters, and the two electrodes are connected to a current source soas to set up between them, an electric discharge, the intensity of whichis gradually in creased. This discharge heats up the glass of the tubeand raises the electrodes to dull red. When the heating is sufficient,the evacuation of the tube is resumed and the discharge is stopped whilecontinuing the pumping, until a fairly good vacuum is obtained, so as toeliminate the gases which have been released from the electrodes andfrom the inner wall of the tube, as well'as the rest of the helium whichwas introduced.

In the case of an electrode designed for a current of 0.1 ampere innormal operation, the intensity of the discharge used for the abovetreatment may for example increase from 0.1 to about 0.3 ampere theduration of the treatment being about three minutes. Such a discharge isless powerful than that which would be required to be used if theelectrodes did not include any lanthanum but it is suflicient in thecircumstances however, since the lanthanum will then absorb whatevertraces there may be of harmful. gas. There are several advantages inemploying a relatively weak discharge. The equipment required formanufacture is simplified and the fluorescent material which possiblycoats the tube is affected but very slightly by the treatment. Theadvantages of use of helium in the treatment have been indicated above.

Discharge tubes, the electrodes of which have been treated in the mannerabove described may have any atmosphere used normally in cold cathodetubes and may contain nitrogen or not.

The piece of lanthanum may be changed to pieces comprising mainlylanthanum or other rare earth metals or alloys.

What I claim is:

In a process for manufacturing a cold and hollow electrode for anelectric discharge device having a gas and mercury vapor atmosphere,said electrode comprising at least one small metal piece fixed to theinner surface of the hollow of said electrode, said metal comprisingmainly at least one rare-earth metal in a metallic condition, the stepof subjecting the electrode provided with said piece and placed in anenclosure'to electric discharges in an atmosphere of helium with anabsolute pressure of a few millimeters of mercury when cold, the heliumcontaining at most a relatively small proportion andthe intensity ofsaid dischargesbeing at least equal to the normal working intensity ofthe electrode, and the subsequent step of pumping said enclosure.

References Cited in the file of this patent of impurities, especially asregards the oxygen content 10 2,456,968

UNITED STATES PATENTS Longini Dec. 21, .1948

